The present invention relates to a cooling system for a gas turbine and particularly relates to an integrated steam/air cooling system for a combined cycle turbine and a method of operating the system.
The traditional approach for cooling gas turbine blades and nozzles is to extract air from a source at a sufficiently high pressure, e.g., by extracting air from the intermediate and last stages of the gas turbine compressor. A series of internal flow passages are typically used to achieve the desired mass flow objectives for cooling the turbine blades; whereas, for nozzles, cooling air is supplied and controlled through external piping. The cooling air flow circuits bypass the combustors where heat is supplied to the thermodynamic cycle. Thus, the diverted coolant air does not receive energy directly from the combustors and does not completely expand through the turbine. This arrangement represents parasitic losses to the turbine output and degrades overall performance efficiency.
Steam cooling in reheat gas turbines has been previously discussed, e.g., see U.S. Pat. Nos. 4,314,442 and 4,565,490 to Rice. Steam cooling has also been discussed in a report by the Electric Power Research Institute, Report No. RP2620-1, entitled "Future Gas Turbine Development Options Definition Study," dated June 1987. This report describes the anticipated performance improvement for steam cooling from a thermodynamic cycle analysis perspective. In the context of that report, the steam cooling supply requirements included a very high pressure source, i.e., on the order of 1840 psia, because it was then believed that such high pressure was needed to overcome circuit friction losses, as well as adverse rotational and centrifugal field forces associated with that proposed closed cooling circuit configuration.
In a combined cycle operation, steam at several pressure and temperature levels is readily available. Coolant air in a gas turbine can be replaced by steam, which is the better cooling medium. Moreover, the problem of degradation of thermal efficiency associated with air as the cooling medium is ameliorated as the transition from air to steam cooling is performed. By using steam as coolant, it is also possible to increase the firing temperatures in the gas turbine cycle.
As set forth in my prior application, identified above, steam and air cooling are integrated in a combined cycle system where primary cooling is provided by steam and off-design operating conditions, e.g., start-up, is provided by air. That is, the gas turbine is operated under normal conditions using steam cooling and has available a backup for operational off-design cooling using air, for example, during start-up or an abrupt failure in the supply of steam. In accordance with that invention, an existing air-cooled gas turbine is modified to change over from operational air cooling to steam cooling. Thus, cooling flow distributions, particularly in the first and second-stage turbine blades and second-stage nozzles, require necessary modification to accommodate steam cooling.
More particularly, second-stage nozzle vanes and first-stage turbine blades are designed specifically to take advantage of the thermal efficiencies of steam cooling. In the second-stage nozzle, a pair of pipes or tubes extend from a manifold coupled to a suitable source of steam from the combined cycle operations and extend through the nozzle vanes and the diaphragm associated with the nozzle vanes. The inner surface of the diaphragm seals with the outer surface of a spacer in a conventional manner, the spacer being carried for rotation with and between the wheels mounting the first and second-stage turbine blades. The spacer defines a pair of chambers with the first and second-stage turbine wheels. Coolant steam passing through the nozzle vanes and through the diaphragm communicates with the chambers and with inlet ports for passage through the first and second-stage turbine blades, as described hereinafter.
Further, discrete inserts envelop and encompass each of the tubes through the nozzle vanes. Each insert is provided with a plurality of apertures for flowing air supplied to the space between the steam carrying tube and the insert outwardly into a cavity defined between the insert and the walls of the nozzle vane. The air cools the nozzle vane and exits the vane both through a series of apertures in its trailing edge and into a chamber within the diaphragm for exit in opposite axial directions into the gas flow through the turbine. The tubes conducting the steam have ribs about their external surfaces to improve the heat transfer relation between the steam within the tubes and the air flowing within the inserts. The external surfaces of the inserts are provided with ribbing, preferably spiral or helical, to direct the flow to the trailing edges and the diaphragm. In operation, the heat transfer between the steam and air lowers the temperature of the steam and increases the temperature of the air. The air flow, however, is expanded and cooled upon passing through the apertures in the insert for cooling impingement against the inside surfaces of the nozzle vanes.
The steam flows through the tubes and diaphragm and through the seal between the diaphragm and the spacer. Preferably, the seal is a labyrinth-type seal with multiple projecting teeth. In accordance with that invention, injector nozzles are spaced one from the other circumferentially about the sealing surface of the spacer. The steam flows from the diaphragm between the adjacent teeth of the labyrinth seal for flow through the injector nozzles in the spacer. The nozzles are shaped to accelerate the flow of steam into the chambers on opposite sides of the spacer.
Coolant steam for the first and second turbine stages is additionally inlet from a location adjacent the shaft of the turbine into the areas between the first and second-stage turbine wheels. Passages are provided through the spacer to enable the steam to enter the chambers. Thus, this inner steam flow passes radially outwardly by centrifugal force to mix with the steam input to the chambers from the tubes of the nozzle stage and the injector nozzles of the spacer. This combined steam flows through and cools the turbine blades of the first and second stages.
In a further aspect of the invention disclosed in my prior application, each first-stage turbine blade includes a serpentine cooling arrangement including four coolant circuits: two single-pass radially outwardly directed passages adjacent the leading and trailing edges of the blade and two intermediate three-pass, forward and aft circuits. The inlet ports for the serpentine passages are through the pedestals mounting the turbine blades. With respect to the forward and aft intermediate circuits, the respective inlet ports are located in the root portion of the blade and the flow of steam is through passages first directed radially outwardly toward the tip portion, then radially inwardly toward the root portion and finally radially outwardly toward the tip portion to exit the turbine blade substantially medially of the blade at its tip portion. The steam therefore flows in serpentine fashion from adjacent the leading and trailing edges in opposite axial directions toward a mid-portion of the turbine blade. Thus, the steam which has collected the most heat from the blade advantageously exits the blade at a location which has the lowest metal temperature.
The leading edge circuit flows steam radially outwardly between an inlet port at the root portion of the blade and an outlet at the tip portion and through a plurality of radially spaced apertures opening into a recess on the leading edge of the blade. That recess is located along the stagnation or pitch area of the blade which is the area of highest blade temperature during operation. The recess contains a porous material, such as woven wire mesh of high density, whereby steam from the first leading edge circuit flows through the apertures into the recess through the mesh for transpiration cooling. The trailing edge circuit flows steam from an inlet port adjacent the root portion of the blade to an outlet adjacent the tip portion, as well as through a series of apertures radially spaced along the trailing edge of the blade.
Additionally, on the pressure side of each vane, there is provided a series of bleed film cooling holes radially spaced along the blade and in communication preferably with the first passageway of the forward intermediate circuit for supplying film cooling steam along the pressure surface of the vane. Film cooling is provided because steam has superior radiant properties, e.g., absorbtivity and emissivity, and absorbs much of the radiant energy and emits this energy at a lower intensity. On the pressure side of the vane, there is also provided a series of bleed film cooling holes radially spaced along the blade, preferably in communication with the final passageway of the aft intermediate circuit. The location of these bleed film cooling holes between the leading and trailing edges of the blade on the suction side is selected because the boundary layer thickens along this area. The boundary layer increases the convective thermal load on the part. By reducing the boundary layer by thin film cooling, the convective thermal load on the part is reduced.
The second-stage turbine blades are each provided with a plurality of straight-through radial passages for passing cooling steam radially outwardly to the blade tips. Each turbine stage has a steam collection shroud adjacent the tips of the blades for collecting the cooling steam.
An air cooling system is integrated with the steam cooling system just described. To accomplish this, a rotating nozzle collar is provided on the inner circumference of the first-stage wheel. Fixed and movable valve structures are mounted about the shaft. The valve is normally closed to prevent air under pressure from the compressor from flowing radially outwardly into the spaces between the wheels and spacer and into the chambers. During start-up or off-design operation, for example, when steam pressure is not available or lost, the solenoid is actuated to open the valve to provide air under pressure into those areas for flow through the inlet ports of the first and second-stage turbine blades to effect cooling. After start-up or when the air cooling is generally not needed, the solenoid closes the valve to prevent air from entering those spaces.
The present invention provides apparatus and methods for integrating steam cooling with existing air cooling designs for gas turbines whereby transitions can be made between air cooling, steam cooling, combinations of air/steam cooling and for all operating conditions of the gas turbine in a manner to obtain full thermodynamic advantage and cycle efficiency. Thus, thermodynamic losses are eliminated or minimized, while, concurrently, steam cooling permits higher firing temperatures for higher machine output ratings. Additionally, combustor emission controls are improved by affording more air and steam in the combustion process without effect on other operational components of the turbine. For example, and still further, thermal low-cycle fatigue of turbine rotor wheels occurs as a result of temperature gradients induced across the wheels caused by cooling air flow around the turbine wheels. By replacing the cooling air with steam, thermal gradients are reduced and life expectancy of the turbine rotor wheels is extended.
The present invention provides a system for integrating the steam cooling features of the first and second stage turbine blades and second stage nozzles set forth in my prior application, with existing air cooling circuits in current gas turbine designs. Moreover, the system integrates air and steam cooling for all modes of operation of the turbine, for example, affording a smooth transition from air cooling to steam cooling during start-up when steam becomes available, steam cooling during normal operations with the option of affording additional air cooling, and with additional cooling afforded by air cooling during periods of abnormal operation, e.g., when particularly hot parts of the turbine blades are detected. Consequently, the present invention affords an integrated steam/air cooling system for gas turbines having better cycle efficiency, higher firing temperatures, enhanced turbine cooling, flexibility in combustion emission control and enhancement of turbine rotor wheel low-cycle fatigue life.
In a preferred embodiment according to the present invention, there is provided an integrated steam/air cooling system for a gas turbine comprising a pair of axially spaced, rotatable turbine stages, each having a plurality of turbine blades for disposition in a gas flow through the turbine, at least certain of the turbine blades each having at least one interior passage, a nozzle stage between the turbine stages and including a plurality of nozzle vanes for disposition in the gas flow through the turbine, at least certain of the vanes each having at least one interior passage. Means are provided for supplying cooling air to the interior passages to air cool the turbine and for supplying steam to the interior passages to steam cool the turbine. Means are cooperable with the cooling air supply means and the steam supply means for effecting a transition between air cooling the turbine and steam cooling the turbine.
In a further preferred embodiment according to the present invention, in a gas turbine having a pair of axially spaced, rotatable turbine stages, each having a plurality of turbine blades for disposition in a gas flow through the turbine with at least certain of the turbine blades each having at least one interior passage, a nozzle stage between the turbine stages and including a plurality of nozzle vanes for disposition in the gas flow through the turbine with at least certain of the vanes each having at least one interior passage, a method of operating a cooling system for the gas turbine including the steps of initially supplying cooling air to the interior passages of said turbine blades to air cool the turbine during start-up of the turbine, thereafter supplying steam to the interior passages of said turbine blades to steam cool the turbine during normal operation of the turbine after turbine start-up and means cooperable with the cooling air supply means and the steam supply means for effecting a transition between air cooling the turbine and steam cooling the turbine.
Accordingly, it is a primary object of the present invention to provide a novel and improved steam/air cooling system for gas turbines.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.